1. Field of the Invention
The present invention relates to a laser processing method for processing a surface of a processing target using laser light, and specifically to a laser processing method preferably usable for ablating a processing target including an organic substance so as to form a flat face which is parallel or inclining with respect to an optical axis of the laser light.
2. Description of the Related Art
One method for processing an organic substance on a substrate by etching uses an ablation function of a laser beam such as, for example, an excimer laser beam.
FIG. 14 shows a schematic view illustrating a structure of a laser processing apparatus 200 usable for performing laser processing. The laser processing apparatus 200 is used for, for example, forming a recessed portion in a processing target 15. The processing target 15 includes an organic sheet formed of, for example, polycarbonate (PC) or polyethylene terephthalate (PET) which is degraded when irradiated with light such as a laser beam. The laser processing apparatus 200 includes an X-Y stage 16 on which the processing target 15 is placed, and a laser oscillator 11 for emitting an excimer laser beam 12 toward the processing target 15. The excimer laser beam 12 emitted by the laser oscillator 11 is provided with a prescribed pattern through a projection mask 13, then is reduced in cross-sectional area by an objective optical system 14, and is directed toward the processing target 15 fixed on the X-Y stage 16.
FIG. 6 is a plan view of the projection mask 13. The projection mask 13 is formed of a glass plate and a metal film provided on the glass plate so as to form a light shielding area 13b. An area of the glass plate which is not covered with the metal film is a rectangular light transmitting area 13a through which the excimer laser beam 12 is allowed to be transmitted.
A spot 21c of the excimer laser beam 12 on the projection mask 13 is elliptical. The spot 21c on the projection mask 13 covers the light transmitting area 13a so that the entirety of the light transmitting area 13a is uniformly irradiated with the excimer laser beam 12. In FIGS. 6, 7 and 8, arrow X represents a direction of longer sides 21a of the rectangular light transmitting area 13a, and arrow Y represents a direction of shorter sides 21b of the rectangular light transmitting area 13a. In this specification, the direction indicated by arrow X will be described as the xe2x80x9cX directionxe2x80x9d, and the direction indicated by arrow Y will be described as the xe2x80x9cY directionxe2x80x9d.
The excimer laser beam 12 which is transmitted through the light transmitting area 13a of the projection mask 13 is reduced in cross-sectional area by the objective optical system 14 and collected on the processing target 15 fixed on the X-Y stage 16. Thus, an image of the rectangular light transmitting area 13a is projected on the processing target 15. The image on the processing target 15 reflects the reduction ratio of the objective optical system 14. A surface of the processing target 15 irradiated with the excimer laser beam 12 is ablated with the excimer laser beam 12. As a result, a recessed portion defined by faces parallel to an optical axis of the excimer laser beam 12 is formed in the processing target 15.
A face inclining with respect to the optical axis of the excimer laser beam 12 can be formed in the processing target 15 by moving the processing target 15 while being irradiated with the excimer laser beam 12.
With respect to FIG. 12, a method for producing the inclining face will be described.
The processing target 15 includes a substrate 15a and an organic sheet 15b bonded to the substrate 15a. For irradiating the processing target 15 with the excimer laser beam 12, conditions for ablating only the organic sheet 15b are used. The excimer laser beam 12 transmitted through the rectangular light transmitting area 13a of the projection mask 13 is directed toward the processing target 15. In this state, the processing target 15 is moved in the direction of arrow C shown in FIG. 12 at a constant speed. A surface of the substrate 15a is perpendicular to the optical axis.
While the processing target 15 is moved in this manner, the irradiation of the excimer laser beam 12 is stopped. Therefore, the total amount of the excimer laser beam 12 received by a front portion of the processing target 15 is different from the total amount of the excimer laser beam 12 received by a rear portion of the processing target 15. The terms xe2x80x9cfrontxe2x80x9d and xe2x80x9crearxe2x80x9d are defined with respect to the direction in which the processing target 15 is moved. As a result of the above-mentioned difference in the total amount of received excimer laser beam 2, the processing target 15 is etched to a different degree in the front portion compared to the rear portion. Therefore, the inclining face which inclines downward from the rear portion toward the front portion of the processing target 15 is formed. A face inclining at any angle can be formed by adjusting the intensity of the excimer laser beam 12 and the moving speed of the processing target 15.
FIG. 7 shows a profile 29 (solid line) of the recessed portion obtained by ablating the surface of the processing target 15 by the laser processing apparatus 200. Since the entirety of the light transmitting area 13a is irradiated with the excimer laser beam 12, the entirety of the profile 29 is wave-shaped, as opposed to an ideal profile 30 (dashed line) which is formed of four straight sides.
The reason why the profile 29 is wave-shaped is because the excimer laser beam 12 transmitted through the light transmitting area 13a of the projection mask 13 is diffracted by edges (i.e., both of the longer sides 21a and the shorter sides 21b; see FIG. 6) of the light transmitting area 13a. 
FIG. 8 shows a light intensity distribution of the excimer laser beam 12 irradiating the surface of the processing target 15 after being transmitted through the light transmitting area 13a. Since the excimer laser beam 12 is diffracted by the edges of the light transmitting area 13a, the light intensity received by the surface of the processing target 15 is not uniform, but portions having a higher light intensity than the rest of the surface appear in a lattice pattern as shown in FIG. 8. Since these portions are ablated more strongly than the rest of the surface the entirety of the profile 29 is wave-shaped.
FIG. 9 is a graph illustrating light intensity distributions of the excimer laser beam 12 irradiating the surface of the processing target 15 along the X direction. A solid line 9a represents a light intensity distribution actually obtained by the laser processing apparatus 200. A dashed line 9b represents a light intensity distribution obtained when the excimer laser beam 12 is not diffracted by the edges of the light transmitting area 13a. The solid line 9a in FIG. 9 corresponds to the lattice shown in FIG. 8. The solid line 9a has peaks having a light intensity level of higher than 1 (referred to as xe2x80x9covershootxe2x80x9d) at positions corresponding to the vicinity of the shorter sides 21bof the light transmitting area 13a (FIG. 6). In addition, the solid line 9a fluctuates in a central portion thereof.
Since it is substantially unavoidable that the light is diffracted at the edges of the light transmitting area 13a, it is difficult to form a recessed portion defined by flat faces as shown by the dashed line 30 in FIG. 7. The inclining face shown in FIG. 12 is also wave-shaped, and it is difficult to form a flat inclining face for the same reason.
Japanese Laid-Open Publication No. 9-206974 discloses a method for improving a light intensity distribution characteristic of a laser beam irradiating a processing target after being transmitted through a light transmitting area of a projection mask. FIG. 10 is a schematic view of a laser processing apparatus 300 disclosed in Japanese Laid-Open Publication No. 9-206974.
Referring to FIG. 10, the laser processing apparatus 300 includes an irradiation optical system IL for uniformly irradiating a projection mask 56 with components of laser light in a superimposing manner. The irradiation optical system IL includes an excimer laser oscillator 51, a beam shaping optical system 52, a fly-eye lens 53 as a homogenizer HN, an aperture 54, and a condenser lens 55. A laser beam 61 emitted by the excimer laser oscillator 51 is enlarged in cross-sectional area by the beam shaping optical system 52 and is directed toward the fly-eye lens 53. The fly-eye lens 53 includes a plurality of lens elements each having a longitudinal axis parallel to an optical axis AX of the laser beam 61. Components of the laser beam 61 coming out of the fly-eye lens 53 reach the condenser lens 55 through the aperture 54 and are collimated by the condenser lens 55. The collimated components of light irradiate the projection mask 56 in a superimposing manner. A light spot of the laser beam 61 on the projection mask 56 covers and thus uniformly irradiates the entirety of a rectangular light transmitting area of the projection mask 56.
The laser beam 61 transmitted through the light transmitting area of the projection mask 56 is collected on a processing target 60 by an imaging optical system OS, which includes two lenses 57 and 58 and an aperture 59. A recessed portion having a pattern corresponding to the light transmitting area of the projection mask 56 is formed in the processing target 60 by ablation provided by the laser beam 61.
Where a numerical aperture of the irradiation optical system IL is NAc and a numerical aperture of the imaging optical system OS is NAo, the coherence factor "sgr" is defined by expression (1).
"sgr"=NAc/NAoxe2x80x83xe2x80x83(1)
FIGS. 11A and 11B show light intensity distributions of the excimer laser beam 12 irradiating the processing target 60 along the X direction. A solid line 111 in FIG. 11A shows a light intensity distribution obtained when the coherence factor "sgr" is 0.2, and a solid line 113 in FIG. 11B shows the light intensity distribution obtained when the coherence factor a is 0.7. A dashed line 112 in FIG. 11A and a dashed line 114 in FIG. 11B each show a light intensity distribution obtained when the laser beam 61 is not diffracted by the edges of the light transmitting area.
The solid line 113 in FIG. 11B exhibits smaller peaks (smaller overshoot portions) and fluctuates less in the central portion than the solid line 111 in FIG. 11A. It is appreciated that an increase in the coherence factor a prevents the intensity of the laser beam from increasing in portions of the surface of the processing target 60 corresponding to the edges of the light transmitting area of the projection mask 56. The increase in the coherence factor a also alleviates fluctuations in the intensity of the laser beam in a portion of the surface of the processing target 60 corresponding to a central area of the light transmitting area of the projection mask 56. When the coherence factor "sgr" is 0.6 or more, a recessed portion defined by flat faces parallel to the optical axis AX (FIG. 10) can be formed in the processing target 60 at relatively high precision.
However, it is still difficult to completely remove the influence of the diffraction of the laser beam by the edges of the light transmitting area as can be appreciated from FIG. 11B. Thus, it is difficult to completely prevent a local increase in the intensity of the laser beam. Formation of a flat face in a processing target, whether parallel or inclining with respect to an optical axis of a laser beam, has not been realized.
A laser processing method according to the present invention includes the steps of irradiating a projection mask having a light transmitting area, for allowing a laser beam to be transmitted therethrough, with the laser beam; and irradiating a processing target with the laser beam transmitted through the light transmitting area. A spot of the laser beam on the projection mask is shaped so as to irradiate a portion in the vicinity of first edges of the light transmitting area, the first edges extending in one direction, and so as not to irradiate a portion in the vicinity of second edges of the light transmitting area, the second edges extending in a second direction which is different from the first direction.
In one embodiment of the invention, the light transmitting area of the projection mask has a shape of a rectangle which extends in the first direction.
In one embodiment of the invention, the second edges of the light transmitting area are shorter sides of the rectangle.
In one embodiment of the invention, the laser processing method further includes the step of moving the processing target in the first direction.
In one embodiment of the invention, the laser processing method further includes the step of moving the processing target in the second direction.
In one embodiment of the invention, the laser processing method further includes the step of reciprocating the processing target in the first direction concurrently with moving the processing target in the second direction.
Thus, the invention described herein makes possible the advantages of providing a low-cost laser processing method for relatively easily controlling diffraction of a laser beam at an edge of a light transmitting area of a projection mask so as to guarantee formation of a flat face in a processing target, whether parallel or inclining to an optical axis of a laser beam.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.